Pain sensitization and degenerative changes are associated with aberrant plantar loading in patients with painful knee osteoarthritis.

OBJECTIVES: This study focused on the biomechanical implications of knee osteoarthritis (OA) and the association with pain. The plantar loading force distribution of the foot was determined and correlated to degenerative knee changes, function, pain intensity, and pain sensitization. METHOD: Knee OA patients (n = 34) with moderate and mild knee pain were characterized and compared to matched controls (n = 16). The Plantar Foot Posture Index (FPI) and mean maximum plantar forces were determined by pressure-sensitive insoles. Pain intensity and function were assessed by the Western Ontario and McMaster Universities Arthritis Index (WOMAC) and the Brief Pain Inventory (BPI). Local knee pain sensitization was assessed by pressure pain thresholds (PPTs) from eight knee locations. Spreading sensitization was assessed by PPTs from two extra-segmental test sites. Temporal summation to repeated pressure stimulation (knee and extra-segmental stimulation) and conditioning pain modulation (CPM) were assessed, representing central pain mechanisms. RESULTS: The maximum force (MF) applied by the medial forefoot correlated to knee PPTs (r = 0.524, p < 0.001), CPM potency (r = 0.532, p < 0.001), and BPI (r = -0.325, p < 0.05) and WOMAC scores (pain r = -0.425, p < 0.01; stiffness r = -0.386, p < 0.01; function r = -0.378, p < 0.05). The MF applied by the medial hindfoot correlated negatively to scores on the FPI (r = -0.394, p < 0.01) and the Kellgren-Lawrence (K-L) grading scale (r = -0.330, p < 0.05). The MF applied by the medial forefoot correlated to extra-segmental PPTs (r = 0.554, p < 0.001) and the potency of CPM (r = 0.561, p < 0.0001). The MF applied by the lateral hindfoot correlated negatively to the PPT assessed extra-segmentally (r = -0.367, p < 0.05) and positively to CPM potency (r = 0.322, p < 0.05). CONCLUSIONS: This study shows that mean maximum plantar foot force distribution in patients with painful knee OA is associated with specific pain mechanisms, function, radiological findings, and pain intensity.

A mechanism-based pain sensitivity index to characterize knee osteoarthritis patients with different disease stages and pain levels.

BACKGROUND: In a cohort of well-characterized patients with different degrees of knee osteoarthritis (OA) and pain, the aims were to utilize mechanism-based quantitative sensory testing (QST) to (1) characterize subgroups of patients; (2) analyse the associations between clinical characteristics and QST; and (3) develop and apply a QST-based knee OA composite pain sensitivity index for patient classification. METHODS: Two hundred seventeen OA pain patients and 64 controls were included. Kellgren and Lawrence (KL) grading scores were obtained, and pressure pain thresholds (PPTs), temporal summation of pain to repeated painful pressure stimulation and conditioning pain modulation (CPM) were assessed. Associations between pain score/area/duration, radiological findings and QST-related parameters were analysed. A pain sensitivity index was developed and applied based on PPT, temporal summation and CPM. z-Score, as statistical tool, was calculated for statistically comparing the pain index of a single patient with a healthy control group. RESULTS: High knee pain associated with low KL grade showed particular signs of pain sensitization. Patients showed significant associations between clinical knee pain intensity/duration and lowering of knee PPTs (p < 0.01), facilitation of temporal summation (p < 0.01), reduction of CPM function (p < 0.01) and high pain sensitivity index (p < 0.01). The index classified 27-38% of the OA patients and 3% of the controls as highly sensitive with no association to KL. The index increased for high knee pain intensities and long pain duration. CONCLUSIONS: Radiological scores, contrary to clinical pain intensity/duration, were poorly associated with QST parameters. The pain sensitivity index could classify OA patients with different degrees of OA and pain.